Twofold Screw Axis Research Articles

Coexistence of topological node surface and Dirac fermions in phonon-mediated superconductor YB2C2.

The interaction between nontrivial topology and superconductivity in condensed matter physics has attracted tremendous research interest as it could give rise to exotic phenomena. Herein, based on first-principles calculations, we investigate the electronic structures, mechanical properties, topological properties, dynamic stability, electron-phonon coupling (EPC), and superconducting properties of the synthesized real material YB2C2. It is a tetragonal structure with P4/mbm symmetry and exhibits excellent stability. The calculated electronic band structures reveal that a zero-dimension (0D) Dirac point and two-dimensional (2D) nodal surface coexist near the Fermi level. A spin-orbit coupling (SOC) Dirac point with the topological Fermi arc is observed on the (001) surface. These nodal surfaces are protected by a two-fold screw axis and time-reversal symmetry. Based on the Bardeen-Cooper-Schrieffer theory, the superconducting transition temperature (Tc) in the range 1.25-4.45 K with different Coulomb repulsion constant μ* for YB2C2 is estimated to be consistent with previous experimental results. In addition, the EPC is mainly from the coupling between the dx2-y2 and dz2 orbitals of the Y atom and low-energy phonon modes. The presence of superconductivity and nontrivial topological surface state in YB2C2 suggests that it may be a candidate material for topological superconductors.

Crystal structures of the complexes containing macrocyclic cations [M(cyclam)]2+ (M = Ni, Zn) and tetra-iodido-cadmate(2-) anion.

The asymmetric units of the isostructural compounds (1,4,8,11-tetra-aza-cyclo-tetra-decane-κ4 N)nickel(II) tetra-iodido-cadmate(II), [Ni(C10H24N4)][CdI4] (I), and tri-iodido-1κ3 I-μ-iodido-(1,4,8,11-tetra-aza-cyclo-tetra-decane-2κ4 N)cad-mium(II)zinc(II), [CdZnI4(C10H24N4)] (II) (C10H24N4 = 1,4,8,11-tetra-aza-cyclo-tetra-decane, cyclam, L), consist of the centrosymmetric macrocyclic cation [M(L)]2+ [M = NiII or ZnII] with the metal ion lying on a twofold screw axis, and the tetra-iodo-cadmate anion [CdI4]2- located on the mirror plane. In I, the anion acts as an uncoordinated counter-ion while in II it is bound to the ZnII atom via one of the iodide atoms, thus forming an electroneutral heterobimetallic complex [Zn(L)(CdI4)]. The NiII and ZnII ions are coordinated in a square-planar manner by the four secondary N atoms of the macrocyclic ligand L, which adopts the most energetically stable trans-III conformation. The [CdI4]2- anions in I and II are structurally very similar and represent slightly deformed tetra-hedrons with average Cd-I bond lengths and I-Cd-I angles of ca 2.79 Å and 109.6°, respectively. The supra-molecular organization of the complexes under consideration in the crystals is very similar and is determined by the hydrogen-bonding inter-actions between the secondary amino groups of the ligand L in the [M(L)]2+ cations and iodide atoms of the [CdI4]2- anion. In particular, the alternating cations and anions form chains running along the b-axis direction that are arranged into di-periodic sheets oriented parallel to the (101) and (01) planes. Because both kinds of sheets are built from the same cations and anions, this feature provides the three-dimensional coherence of the crystals of I and II.

Syntheses, crystal structures and thermal properties of catena-poly[cadmium(II)-di-μ-bromido-μ-pyridazine-κ2 N 1:N 2] and catena-poly[cadmium(II)-di-μ-iodido-μ-pyridazine-κ2 N 1:N 2

The reactions of cadmium bromide and cadmium iodide with pyridazine (C4H4N2) in ethanol under solvothermal conditions led to the formation of crystals of [CdBr2(pyridazine)] n (1) and [CdI2(pyridazine)] n (2), which were characterized by single-crystal X-ray diffraction. The asymmetric units of both compounds consist of a cadmium cation located on the inter-section point of a twofold screw axis and a mirror plane (2/m), a halide anion that is located on a mirror plane and a pyridazine ligand, with all atoms occupying Wyckoff position 4e (mm2). These compounds are isotypic and consist of cadmium cations that are octa-hedrally coordinated by four halide anions and two pyridazine ligands and are linked into [100] chains by pairs of μ-1,1-bridging halide anions and bridging pyridazine ligands. In the crystals, the pyridazine ligands of neighboring chains are stacked onto each other, indicating π-π inter-actions. Larger amounts of pure samples can also be obtained by stirring at room-temperature, as proven by powder X-ray diffraction. Measurements using thermogravimetry and differential thermoanalysis (TG-DTA) reveal that upon heating all the pyridiazine ligands are removed in one step, which leads to the formation of CdBr2 or CdI2.

Nonlinear Optical and Magnetic Properties of FeII-SCN-HgII Isomers: Centrosymmetric Layers and Chiral Networks

Research on isomers is highly desirable due to their prospective role in better understanding of physicochemical properties of similar systems and further development of multifunctional molecular materials. Iron(II) and tetra(thiocyanato)mercury(II) ions self-assembled in the presence of 2-acetylpyridine (2-acpy) excess to form two {[Fe(2-acpy)][Hg(μ-SCN)4]}n isomers: two-dimensional (2D) centrosymmetric layers with folded ring structural motifs (1) and three-dimensional (3D) chiral networks with right- or left-handed {···Fe-NCS-Hg-SCN···}∞ helixes (2). New methods of designing and synthesizing functional thiocyanate-bridged materials have been proposed. In addition, the similarity between 1 and 2 allowed for the description of subtle changes in IR and UV-visible spectra. Moreover, 2 shows spontaneous resolution, and it crystallizes in the noncentrosymmetric space group P21, leading to the occurrence of nonlinear optical activity in circular dichroism studies and second harmonic generation (SHG). At room temperature, the SH susceptibility for powder sample 2 reached 6.0 × 10-11 esu. Ab initio calculations indicated the electric polarization vector and the crystallographic twofold screw axis pass through the aromatic ring. Magnetic studies for 1 and 2 revealed high-spin iron(II) with zero-field splitting at low temperatures. Analysis of magnetic data gave |D| = 37.45 cm-1, |E/D| = 5.59 cm-1, and ⟨g⟩ = 2.15 for 1, |D| = 36.78 cm-1, |E/D| = 4.92 cm-1, and ⟨g⟩ = 2.18 for 2, and information about the orientation of magnetic anisotropy vectors for both compounds.

How Crystal Symmetry Dictates Non-Local Vibrational Circular Dichroism in the Solid State.

Solid-State Vibrational Circular Dichroism (VCD) can be used to determine the absolute structure of chiral crystals, but its interpretation remains a challenge in modern spectroscopy. In this work, we investigate the effect of a twofold screw axis on the solid-state VCD spectrum in a combined experimental and theoretical analysis of P21 crystals of (S)-(+)-1-indanol. Even though the space group is achiral, a single proper symmetry operation has an important impact on the VCD spectrum, which reflects the supramolecular chirality of the crystal. Distinguishing between contributions originating from molecular chirality and from chiral crystal packing, we find that while IR absorption hardly depends on the symmetry of the space group, the situation is different for VCD, where completely new non-local patterns emerge. Understanding the two underlying mechanisms, namely gauge transport and direct coupling, will help to use VCD to distinguish polymorphic forms.

How Crystal Symmetry Dictates Non‐Local Vibrational Circular Dichroism in the Solid State

AbstractSolid‐State Vibrational Circular Dichroism (VCD) can be used to determine the absolute structure of chiral crystals, but its interpretation remains a challenge in modern spectroscopy. In this work, we investigate the effect of a twofold screw axis on the solid‐state VCD spectrum in a combined experimental and theoretical analysis of P21 crystals of (S)‐(+)‐1‐indanol. Even though the space group is achiral, a single proper symmetry operation has an important impact on the VCD spectrum, which reflects the supramolecular chirality of the crystal. Distinguishing between contributions originating from molecular chirality and from chiral crystal packing, we find that while IR absorption hardly depends on the symmetry of the space group, the situation is different for VCD, where completely new non‐local patterns emerge. Understanding the two underlying mechanisms, namely gauge transport and direct coupling, will help to use VCD to distinguish polymorphic forms.

Crystal structure, PIXEL calculations of inter-molecular inter-action energies and solid-state characterization of the herbicide isoxaflutole.

In the isoxaflutole mol-ecule {systematic name: (5-cyclo-propyl-1,2-oxazol-4-yl)[2-(methyl-sulfon-yl)-4-(tri-fluoro-meth-yl)phen-yl]methanone; C15H12F3NO4S}, the 1,2-oxazole and methanone fragments form an almost coplanar unit, whereas the methanone and phenyl mean planes are inclined by an angle of more than 60°. This conformation differs fundamentally from all other known examples of the 1,2-oxazol-4-yl(phen-yl)methanone fragment and is ascribed to the presence of the bulky methyl-sulfonyl para substituent at the phenyl ring. PIXEL calculations reveal that the largest contributions to the stabilization of the crystal persist within a columnar arrangement of mol-ecules along the twofold screw axis and in inter-actions between adjacent columns related by an inversion operation. Both these intra-column and inter-column motifs are dominated by the dispersion energy term but also display additional significant stabilization effects as a result of three short inter-molecular C-H⋯O contacts involving the methane-sulfonyl-O atoms.

1,1'-Methyl-enebis{4-[(E)-2-(pyridin-4-yl)ethen-yl]pyridinium} dibromide dihydrate.

The chevron-shaped cations of the title hydrated salt, C25H22N4 2+·2Br-·2H2O, are arranged in back-to-back alternating directions to form a zigzag ribbon propagating along the [010] direction. Inter-molecular inter-actions comprising these ribbons are π-π inter-actions between the pyridinium and adjacent pyridyl rings, as well as O-H⋯O hydrogen bonding between water molecules and two adjacent pyridyl N atoms. Half of the cation is generated by the mirror plane. The water O atoms, the central C atom and one Br atom are located on this mirror plane while the other Br atom is on a twofold screw axis.

Hypothetical P63/mmc-Type CsCrCl3 Ferromagnet: Half-Metallic Property and Nodal Surface State

This work is focusing on the electronic, magnetic, half-metallic behaviors, and interesting surface state of hypothetical CsCrCl3 ferromagnet with P63/mmc space group that are investigated under the spin-polarized Generalized Gradient Approach (GGA) and spin-polarized GGA plus Hubbard U correction within the framework of the Density Functional Theory (DFT). The calculated total magnetic moment of P63/mmc CsCrCl3 is ~7.397 µB, and the mainly contribution to the total magnetism is coming from the Cr atoms. Based on obtained spin-polarized band structures, we confirmed that this hypothetical CsCrCl3 is a half-metal with 100% spin-polarization. Without spin-orbit coupling, i.e., the spin and the orbit degrees of freedom are dependents, we found that there should be a interesting nodal surface state at kz = plane due to P63/mmc type lattice structure enjoys a two-fold screw axis S2z. Our study can be seen as a strong evidence to exhibit that the protected nodal surface state can be achieved in magnetic materials.

Rac-12-Selena-13,14-di-aza-tri-cyclo-[9.3.0.02,4]tetra-deca-11,13-diene.

The centrosymmetric crystal structure of the title compound, C11H16N2Se, is built up from alternating strands of (R,R)- and (S,S)-enanti-omers. These strands, which propagate along the c-axis direction, are composed of homochiral mol-ecules related to each other by twofold screw axes. The shape of the mol-ecule is an almost planar unit around the selena-diazole ring with a hexa-methyl-ene chain as an arched handle.

One mol-ecule, three crystal structures: conformational trimorphism of N-[(1S)-1-phenyl-eth-yl]benzamide.

The title compound, C15H15NO, is an enanti-opure small mol-ecule, which has been synthesized many times, although its crystal structure was never determined. By recrystallization from a variety of solvent mixtures (pure aceto-nitrile, ethanol-water, toluene-ethanol, THF-methanol), we obtained three unsolvated polymorphs, in space groups P21 and P212121. Form I is obtained from aceto-nitrile, without admixture of other forms, whereas forms II and III are obtained simultaneously by concomitant crystallizations from alcohol-based solvent mixtures. All forms share the same supra-molecular structure, based on infinite C 1 1(4) chain motifs formed by N-H⋯O inter-molecular hydrogen bonds, as usual for non-sterically hindered amides. However, a conformational modification of the mol-ecular structure, related to the rotation of the phenyl rings, alters the packing of the chains in the crystal structures. The orientation of the chain axis is perpendicular and parallel to the crystallographic twofold screw axis of space group P21 in forms I and II, respectively. As for form III, the asymmetric unit contains two independent mol-ecules forming parallel chains in space group P212121, and the crystal structure combines features of monoclinic forms I and II.

Molecular recognition in the product site of cellobiohydrolase Cel7A regulates processive step length.

Cellobiohydrolase Cel7A is an industrial important enzyme that breaks down cellulose by a complex processive mechanism. The enzyme threads the reducing end of a cellulose strand into its tunnel-shaped catalytic domain and progresses along the strand while sequentially releasing the disaccharide cellobiose. While some molecular details of this intricate process have emerged, general structure-function relationships for Cel7A remain poorly elucidated. One interesting aspect is the occurrence of particularly strong ligand interactions in the product binding site. In this work, we analyze these interactions in Cel7A from Trichoderma reesei with special emphasis on the Arg251 and Arg394 residues. We made extensive biochemical characterization of enzymes that were mutated in these two positions and showed that the arginine residues contributed strongly to product binding. Specifically, ∼50% of the total standard free energy of product binding could be ascribed to four hydrogen bonds to Arg251 and Arg394, which had previously been identified in crystal structures. Mutation of either Arg251 or Arg394 lowered production inhibition of Cel7A, but at the same time altered the enzyme product profile and resulted in ∼50% reduction in both processivity and hydrolytic activity. The position of the two arginine residues closely matches the two-fold screw axis symmetry of the substrate, and this energetically favors the productive enzyme-substrate complex. Our results indicate that the strong and specific ligand interactions of Arg251 and Arg394 provide a simple proofreading system that controls the step length during consecutive hydrolysis and minimizes dead time associated with transient, non-productive complexes.

Cations Controlling the Chiral Assembly of Luminescent Atomically Precise Copper(I) Clusters.

Chiral assembly and asymmetric synthesis are critically important for the generation of chiral metal clusters with chiroptical activities. Here, a racemic mixture of [K(CH3 OH)2 (18-crown-6)]+ [Cu5 (St Bu)6 ]- (1⋅CH3 OH) in the chiral space group was prepared, in which the chiral red-emissive anionic [Cu5 (St Bu)6 ]- cluster was arranged along a twofold screw axis. Interestingly, the release of the coordinated CH3 OH of the cationic units turned the chiral 1⋅CH3 OH crystal into a mesomeric crystal [K(18-crown-6)]+ [Cu5 (St Bu)6 ]- (1), which has a centrosymmetric space group, by adding symmetry elements of glide and mirror planes through both disordered [Cu5 (St Bu)6 ]- units. The switchable chiral/achiral rearrangement of [Cu5 (St Bu)6 ]- clusters along with the capture/release of CH3 OH were concomitant with an intense increase/decrease in luminescence. We also used cationic chiral amino alcohols to induce the chiral assembly of a pair of enantiomers, [d/l-valinol(18-crown-6)]+ [Cu5 (St Bu)6 ]- (d/l-Cu5V ), which display impressive circularly polarized luminescence (CPL) in contrast to the CPL-silent racemic mixture of 1⋅CH3 OH and mesomeric 1.

Cations Controlling the Chiral Assembly of Luminescent Atomically Precise Copper(I) Clusters

AbstractChiral assembly and asymmetric synthesis are critically important for the generation of chiral metal clusters with chiroptical activities. Here, a racemic mixture of [K(CH3OH)2(18‐crown‐6)]+[Cu5(StBu)6]− (1⋅CH3OH) in the chiral space group was prepared, in which the chiral red‐emissive anionic [Cu5(StBu)6]− cluster was arranged along a twofold screw axis. Interestingly, the release of the coordinated CH3OH of the cationic units turned the chiral 1⋅CH3OH crystal into a mesomeric crystal [K(18‐crown‐6)]+[Cu5(StBu)6]− (1), which has a centrosymmetric space group, by adding symmetry elements of glide and mirror planes through both disordered [Cu5(StBu)6]− units. The switchable chiral/achiral rearrangement of [Cu5(StBu)6]− clusters along with the capture/release of CH3OH were concomitant with an intense increase/decrease in luminescence. We also used cationic chiral amino alcohols to induce the chiral assembly of a pair of enantiomers, [d/l‐valinol(18‐crown‐6)]+[Cu5(StBu)6]− (d/l‐Cu5V), which display impressive circularly polarized luminescence (CPL) in contrast to the CPL‐silent racemic mixture of 1⋅CH3OH and mesomeric 1.

The Importance of CH···X (X = O, π) Interaction of a New Mixed Ligand Cu(II) Coordination Polymer: Structure, Hirshfeld Surface and Theoretical Studies

In this study, a new equimolar (1:1:1) mixed ligand Cu(II) polymer, [Cu(IDA)(ImP)]n (1) with iminodiacetato (IDA) and imidazo[1,2-a]-pyridine (ImP) was synthesized and characterized by single crystal X-ray diffraction analysis. X-ray crystallography reveals that compound (1) consists of polymeric zigzag chain along [010] the carboxylate carbonyl oxygen atom by two-fold symmetry screw axis. The solid-state structure is stabilized through C–H···O hydrogen bonds and C–H···π interactions that lead the molecules to generate two-dimensional supramolecular assemblies. The intricate combinations of hydrogen bonds and C–H···π interactions are fully described along with computational studies. A thorough analysis of Hirshfeld surface and fingerprint plots elegantly quantify the interactions involved within the structure. The binding energies associated with the noncovalent interactions observed in the crystal structure and the interplay between them were calculated using theoretical DFT calculations. Weak noncovalent interactions were analyzed and characterized using Bader’s theory of ‘‘atoms-in-molecules’’ (AIM). Finally, the solid-state supramolecular assembly was characterized by the “Noncovalent Interaction” (NCI) plot index.

Hourglasslike nodal net semimetal in Ag2BiO3

Based on first-principles calculation and analysis of crystal symmetries, we propose a kind of hourglass-like nodal net (HNN) semimetal in centrosymmetric Ag2BiO3 that is constructed by two hourglass-like nodal chains (HNCs) at mutually orthogonal planes in the extended Brillouin zone (BZ) when the weak spin-orbit coupling (SOC) mainly from the 6s orbital of Bi atoms is ignored. The joint point in the nodal net structure is a special double Dirac point located at the BZ corner. Different from previous HNN [Bzdusek et al., Nature 538, 75 (2016)] where the SOC and double groups nonsymmorphic symmetries are necessary, and also different from the accidental nodal net, this HNN structure is inevitably formed and guaranteed by spinless nonsymmorphic symmetries, and thus robust against any symmetry-remaining perturbations. The Fermi surface in Ag2BiO3 consisting of a torus-like electron pocket and a torus-like hole pocket may lead to unusual transport properties. A simple four-band tight-binding model is built to reproduce the HNN structure. For a semi-infinite Ag2BiO3 , the "drumhead" like surface states with nearly flat dispersions are demonstrated on (001) and (100) surfaces, respectively. If such weak SOC effect is taken into consideration, this HNN structure will be slightly broken, left a pair of hourglass-like Dirac points at the two-fold screw axis. This type of hourglass-like Dirac semimetal is symmetry-enforced and does not need band inversion anymore. Our discovery provides a new platform to study novel topological semimetal states from nonsymmorphic symmetries.

Phonon-induced topological transition to a type-II Weyl semimetal

Given the importance of crystal symmetry for the emergence of topological quantum states, we have studied, as exemplified in $\mathrm{NbNiT}{\mathrm{e}}_{2}$, the interplay of crystal symmetry, atomic displacements (lattice vibration), band degeneracy, and band topology. For the $\mathrm{NbNiT}{\mathrm{e}}_{2}$ structure in space-group 53 (Pmna)---having an inversion center arising from two glide planes and one mirror plane with a twofold rotation and screw axis---a full gap opening exists between two band manifolds near the Fermi energy. Upon atomic displacements by optical phonons, the symmetry lowers to space-group $28(Pma2)$, eliminating one glide plane along $c$, the associated rotation and screw axis, and the inversion center. As a result, 20 Weyl points emerge, including four type-II Weyl points in the \ensuremath{\Gamma}-$X$ direction at the boundary between a pair of adjacent electron and hole bands. Thus, optical phonons may offer control of the transition to a Weyl fermion state.

The silver(I) nitrate complex of the ligand N-(pyridin-2-ylmeth-yl)pyrazine-2-carboxamide: a metal-organic framework (MOF) structure.

The reaction of silver(I) nitrate with the mono-substituted pyrazine carboxamide ligand, N-(pyridin-2-ylmeth-yl)pyrazine-2-carboxamide (L), led to the formation of the title compound with a metal-organic framework (MOF) structure, [Ag(C11H10N4O)(NO3)] n , poly[μ-nitrato-[μ-N-(pyridin-2-ylmethyl-κN)pyrazine-2-carboxamide-κN4]silver(I)]. The silver(I) atom is coordinated by a pyrazine N atom, a pyridine N atom, and two O atoms of two symmetry-related nitrate anions. It has a fourfold N2O2 coordination sphere, which can be described as distorted trigonal-pyramidal. The ligands are bridged by the silver atoms forming -Ag-L-Ag-L- zigzag chains along the a-axis direction. The chains are arranged in pairs related by a twofold screw axis. They are linked via the nitrate anions, which bridge the silver(I) atoms in a μ2 fashion, forming the MOF structure. Within the framework there are N-H⋯O and C-H⋯O hydrogen bonds present.

4-Methyl-N-(4-methylpyridin-2-yl)-N-(3,4,5,6-tetrahydro-2H-pyran-4-yl)pyridin-2-amine

In the title compound, C17H21N3O, the pyridine rings make a dihedral angle of 84.44 (5)°. In the crystal, a C—H...N interaction forms a chain of molecules propagating along the twofold screw axis.

CRYSTAL STRUCTURE OF 2,7-DIMETHOXY-1,8-BIS(1-NAPHTHOYL)NAPHTHALENE: COOPERATIVE HYDROGEN BONDS AND THE ROLE IN THE MOLECULAR PACKING

The aromatic rings in 2,7-dimethoxy-1,8-bis(1-naphthoyl)naphthalene in crystal accumulate non-coplanarly to each other, with the dihedral angle between the terminal naphthalene and the 2,7-dimethoxynaphthalene rings of closer to 90˚ than the homologous compounds, 1,8-dibenzoyl-2,7-dimethoxynaphthalene and 2,7-dimethoxy-1,8-bis(2-naphthoyl)naphthalene. Molecular packing structures of the title compound and the homologous two compounds are classified by screw-sense and orientation of molecular assembly with a two-fold screw axis. In the title compound, right(left)-handed 2 1 helical assemblies are oriented in same direction and aligned to form a plane. The plane is alternately stacked with the counter handed version. On the other hand, right(left)-handed 2 1 helical assemblies are oppositely oriented to each other in the homologous two compounds. In the right(left)-handed 2 1 helical assembly of the title compound, two types of effective C–H…O=C hydrogen bonds are observed, i.e., (sp 3 )C–H…O=C hydrogen bond and (sp 2 )C–H…O=C hydrogen bond. However, either of two types of C–H…O=C hydrogen bonds is found in a right(left)-handed 2 1 helical assembly of the homologous two compounds. Besides, the title compound has (sp 3 )C–H…O=C hydrogen bonds between the right-handed 2 1 helical assembly and the left-handed one, the homologous two compounds have effective pi…pi stacking interactions and (sp 2 )C–H…O=C hydrogen bonds. When two types of C–H…O=C hydrogen bonds cooperatively stabilize the right(left)-handed 2 1 helical assembly, the helical assemblies are oriented in single direction to form a plane. Presence of pi…pi stacking interaction between the right-handed 2 1 helical assembly and the counter handed one apparently mask the effect of C–H…O=C hydrogen bonds to molecular packing to block a plane composed of single-handed 2 1 helical assembly.